There are large worldwide stored inventories of the depleted uranium hexafluoride (UF.sub.6) tailings from past gaseous-diffusion-enrichment plants for nuclear fuel-cycle operations. In the U.S. alone, there are currently .apprxeq.700,000 metric tons of that material stored in U.S DOE facilities at Paducah, Ky., Portsmouth, Ohio and Oak Ridge, Tenn. One conversion process considered in the past by the DOE would use steam for high-temperature hydrolysis of the hexafluoride, to convert it to (a) UO.sub.2 for use in other nuclear programs or for disposal, and (b) anhydrous HF for industrial use. The present invention relates to a low-temperature aqueous process to convert the hexafluoride to UO.sub.2 for use in other nuclear programs, and high-purity calcium fluoride for sale.